Two basic types of optical modulators are commonly used for modulating laser beams in laser applications. One basic type is known as an acousto-optic (AO) modulator. An AO modulator employs as a switching element a crystal having a pressure-sensitive refractive index, i.e., a piezoelectric crystal. Crystal-quartz is favored for most visible and near infrared (NIR) applications. A radio frequency (RF) voltage is applied to one face of the crystal, typically cooperative with an acoustic absorber on an opposite face. The applied RF voltage causes a sound-wave of corresponding frequency to traverse the crystal. This sound-wave induces a periodic variation of refractive index along the wave-direction. This periodic index variation acts as a diffraction grating. This grating diffracts a portion of an input-beam traversing the crystal (perpendicular to the sound wave direction) as an output-beam away from the main-beam direction to whatever application requires a modulated beam. Alternatively, the input-beam may be introduce in the first order direction of the grating and diffracted into the zero order. This is the more efficient mode.
A particular problem with such an AO-modulator is that transmission efficiency into the output-beam is usually less than about 90%. There can also be a switching-speed limited imposed by the time required for the sound wave to traverse the crystal. This can limit switching rates to tens of kilohertz (kHz) or less.
The other basic modular type is known as an electro-optic (EO) modulator. An EO-modulator employs as a switching-element a birefringent crystal, the birefringence of which can be varied by the application of a high voltage (one kilovolt or more), a property known as the linear electro-optic effect or “Pockels effect,” after the discoverer. The crystal is typically used in conjunction with polarization-selective beam-splitters which reflect or transmit an incident beam according to the polarization state (orientation) of the incident beam. The polarization-orientation (determined by the polarizer) of a beam to be modulated is switched through 90° by applying sufficient voltage to the crystal. The beam will be transmitted or reflected by the crystal according to the orientation of the crystal with respect to the beam. Switching speeds of tens of nanoseconds, are possible, depending on voltage-driver and crystal configuration.
One shortcoming of this type of modulator is that polarization-selective beam-splitters have less than 100% efficiency for separating 90°-opposed polarization states. Two-types of polarization-sensitive beam-splitter are typically used. One type is a cemented bi-prism with a reflective multilayer coating at the cemented interface. The materials of the prism and coating layers are selected to have the same refractive index for light polarized in the plane of incidence of the interface. This referred to as p-polarized light by practitioners of the art.
While in theory this should permit 100 percent transmission through the interface, it is rarely achieved consistently in practice. This is because the cemented prisms have residual stress birefringence imposed in manufacturing. This residual birefringence slightly rotates the polarization orientation of light at the interface, which causes transmitted and reflected beams to have the polarization orientation thereof made slightly elliptical.
A second type of polarization-sensitive beam-splitter is known to practitioners of the art as a front-surface polarizer. This splitter has a polarization-selective multilayer coating applied to one surface of a relatively-thin substrate and is typically used at close the Brewster angle to an incident beam. This minimizes effects of residual birefringence in the substrate, and avoids the use of cement which can be a problem in high power applications. The coatings, however, have only a relatively narrow wavelength range of polarization separation and transmission and it is doubtful that greater than 99% transmission of p-polarized radiation could be routinely achieved. Accordingly with either type of beam-splitter, it is doubtful whether a polarization purity (extinction ratio) less than 1×10−2 for a modulated beam can be routinely achieved.
In certain applications, in particular laser-illuminated confocal microscopy with line blanking, an extinction ratio on the order of at least 1×10−3, and preferably on the order of 1×10−5 is desirable. There is a need for an EO-modulator that can achieve such extinction ratios, without sacrifice of switching speed.